The present invention relates generally to a cap nut to be enveloped in a casting and a casting method for enveloping the cap nut in the casting, and more particularly to a cap nut which is installable in and removable from a pattern for sand mold and a casting method for enveloping the cap nut in the casting.
When a component is assembled to a large or heavy casting, a fastening bolt is often used which is inserted into a cap nut that is enveloped in casting material of the casting. For enveloping a cap nut in a cast portion, casting material or molten metal is poured into a sand mold with a bolt installed into a threaded hole of the cap nut and buried in the sand mold. In the casting method, however, the cap nut expands and contracts under the influence of heat of the casting material and cooling after casting. In addition, part of the casting material may enter between the cap nut and the bolt thereby causing seizure. The expansion and contraction of the cap nut and the seizure of the cap nut and the bolt cause the threaded hole of the cap nut to be damaged or deformed. Therefore, it is necessary to form the threaded hole again using a tap after the casting. Thus, additional work for forming the threaded hole of the cap nut has caused a decrease in productivity and an increase in manufacturing cost.
There has been recently proposed an art for solving such problems. For example, Unexamined Japanese Patent Publication No. 2002-192326 discloses a casting method for enveloping a cap nut 40 in the casting by a full mold process as shown in FIGS. 12A through 12C. The cap nut 40 has formed therein a stepped hole 43 that includes a large-diameter portion 43a on the bottom side and a small-diameter portion 43b on the opening side. The cap nut 40 has formed therearound adjacent to the large-diameter portion 43a a circumferential notch 44 having a V-shaped cross-section. With this circumferential notch 44 as the boundary, the cap nut 40 has a cap nut body 41 on the bottom side thereof and a cylindrical extension 42 on the opposite side.
The casting method for the above cap nut 40 will be now described. The cap nut 40 is initially inserted into a support hole 46 that is formed in an evaporative pattern 45 made of, for example, expanded polystyrene. The cap nut 40 is inserted into the support hole 46 to such a depth that the circumferential notch 44 is positioned flush with the surface of the evaporative pattern 45. After the cap nut 40 is fixed relative to the evaporative pattern 45, a bolt 47 is screwed into the large-diameter portion 43a. Thereafter, a sand mold 50 is made so that the sand mold 50 surrounds the evaporative pattern 45 and also that the bolt 47 and the cap nut 40 are partially buried in the sand mold 50. As shown in FIG. 12A, the nut body 40a is supported by the evaporative pattern 45, while the cylindrical extension 50b is buried in the sand mold 50.
The casting material is then poured into the sand mold 50. The casting material runs in the cavity of the sand mold 50 and replaces the evaporative pattern 45, so that the cap nut 40, which is fixed to the sand mold 50 by the bolt 47, is enveloped in the casting material. Because the hole 43 of the cap nut 40 is then closed by the bolt 47, the casting material does not enter into the hole 43.
As shown in FIG. 12B, the resulting casting 49, which is taken out of the sand mold 50 after cooling, includes a cast portion 52 made of the casting material and the cap nut 40 enveloped in the cast portion 52, and the bolt 47 is still inserted in the cap nut 50. As shown in FIG. 12C, the bolt 47 is sheared at the circumferential notch 44. As a result, the bolt 47 and the cylindrical extension 43b of the cap nut 40 are removed, and the body 43a of the cap nut 40 remains in the cast portion 52. The sheared surface adjacent to the hole 43 of the nut body 43a is positioned flush with the surface of the cast portion 52. A component may be fixed to the casting 49 by using a fastening bolt (not shown) inserted into the small-diameter threaded hole 43a of the cap nut 40 that is enveloped in the cast portion 52.
Thus, the above-described casting method contributes greatly to productivity improvement and reduction of manufacturing cost, and the cap nut 40 having the circumferential notch 44 on the outer peripheral surface thereof is appropriate for performing this casting method.
However, this conventional cap nut is used in the full-mold process using an evaporative pattern, but it is not usable in a casting method using a recyclable pattern for sand mold. Additionally, since the conventional cap nut requires the shearing after casting, the outer diameter of the cap nut and the inner diameter of the hole into which the bolt is inserted suitable for the shearing should be determined previously, which inhibits the freedom of establishing the values for such diameters. The sheared surface of the cap nut, which is to be used as a seating face for fitting a component to the casting, tends to be established small in area. Therefore, when a heavy component is to be fitted to the casting, a fastening bolt with a relatively large axial tension cannot be used.
Meanwhile, FIGS. 13A through 13C illustrate another casting method using a resin pattern for sand mold, instead of an evaporative pattern for enveloping a cap nut 40 in the casting. In this casting method, a pattern 58 is used for forming a sand mold 50 and has formed therein a recess 58a that corresponds to a desired position where the cap nut 40 is enveloped. The recess 58a has a tapered inner surface which is formed such that the recess 58a broadens from its bottom toward its opening. A bolt fitting 56 fillable to the recess 58a of the pattern 58 is prepared. The bolt fitting 56 has formed therein a threaded hole 57 that receives therein a bolt 47 as a fixing member. The bolt mold 56 has a truncated cone shape so that its outer peripheral surface fits in the recess 58a. The bolt fitting 56 is also removable from the recess 58a. 
The cap nut 40 is substantially cylindrical and has formed in an exposed surface 41a thereof a threaded hole 53 that extends in the longitudinal direction of the cap nut 40. The cap nut 40 has formed in the periphery thereof engaging grooves 54 as retaining means for preventing the cap nut 40 from falling off from a cast portion. The cap nut 40 is formed with an inclined portion 55 as a rotation preventing means for preventing the cap nut 40 from rotating relative to the cast portion.
This casting method will be now described. Initially, a locknut 48 is installed on the bolt 47. The bolt 47 is inserted into the bolt fitting 56, and then the bolt fitting 56 is fitted in the recess 58a of the pattern 58 to be held in the pattern 58. Then, molding cavity is filled with molding sand in such a way that the molding sand surrounds the pattern 58 and the bolt 47 is buried in the resulting sand mold 50, as shown in FIG. 13A. After the molding sand has been hardened, the pattern 58 is removed from the sand mold 50. Since the bolt 47 is buried in the sand mold 50, the bolt fitting 56 remains in the sand mold 50 after removing the pattern 58. As shown in FIG. 13B, the bolt fitting 56 is removed from the bolt 47. As shown in FIG. 13C, the cap nut 40, which is to be enveloped in the casting, is installed over part of the bolt 47 which protrudes from the surface of the sand mold 50. With the cap nut 40 thus installed, the cap nut 40 is held by the bolt 47 with the exposed surface 41a of the cap nut 40 set in contact with the sand mold 50.
Sequentially, casting material is poured into the cavity of the sand mold 50 and runs therein, so that the cap nut 40 held by the sand mold 50 through the bolt 47 is enveloped in the casting material as shown in FIG. 14A. After the casting material has been cooled, a casting 51 is taken out of the sand mold 50 as shown in FIG. 14B. The casting 51 includes a cast portion 52 and the cap nut 40 enveloped in the cast portion 52, and the bolt 47 is still inserted in and held by the cap nut 40. Removing the bolt 47 from the cap nut 40, the casting 51 is made with the cap nut 40 enveloped therein as shown in FIG. 14C.
The envelopment casting of the cap nut 40 as shown in FIGS. 13A through 14C requires removing the bolt fitting 56 from the bolt and installing the cap nut over the bolt after the pattern 58 is removed from the sand mold 50. Such work is troublesome and time-consuming, which prevents improvement of casting productivity and reduction in manufacturing cost.
The present invention is directed to a cap nut for envelopment casting and also to a method of enveloping the cap nut in a casting which are advantageous in that a repeatable pattern is usable for productivity improvement and casting cost reduction and also that the cap nut can have a sufficiently large seating area because of a higher degree of design of the cap nut.